1. Field of the Invention
The present invention relates to an optical pickup apparatus recording a signal on an optical recording medium or reproducing a recorded signal, an optical recording medium driving apparatus equipped with the same, and a signal recording/reproducing method.
2. Description of the Related Art
Among optical disks serving as optical recording mediums, there has been a disk having two recording layers on one side. Hereinafter, this optical disk is referred to as a dual-layer disk for convenience. When this dual-layer disk is used, reflected light from a target layer which is a target of recording or reproduction out of two recording layers of this dual-layer disk and reflected light from an adjacent layer which is not the target layer enter a detector for photoelectric conversion. In this case, since the power (or light intensity) of the reflected light from the adjacent layer is sufficiently lower than the power of the reflected light from the target layer, the interference with each reflected light in the detector is insignificant. Also in this case, since a main beam from the adjacent layer is not focused on the detector, the beam diameter thereof is increased near the detector (see, for example, Japanese Patent Application Laid-open No. 2005-203090 (paragraph [0098], and so on) (hereinafter, referred to as Patent Document 1)).
Incidentally, in a three-beam method used for tracking control, laser light from a laser light source is separated into three beams: a main beam being zero-order diffracted light and sub-beams being positive and negative first-order diffracted light. The sub-beam is detected by a sub-detector different from the detector for detecting the main beam. A tracking error signal which is a DPP (Differential Push-Pull) signal is obtained by a predetermined calculation formula using push-pull signal values of these main beam and two sub-beams. Generally, the power of the main beam is supposed to be approximately 10 times to 20 times as large as that of the sub-beam.
When this three-beam method is used for the recording or reproduction of the above-mentioned dual-layer disk, the following problem arises. Specifically, the power of the sub-beam from the target layer is relatively close to the power of the main beam from the adjacent layer, and hence the interference or the like becomes marked. This interference causes variations in the power of light entering the sub-detector. Further, since an objective lens moves in a tracking direction by tracking control, the interference portion thereof oscillates, and consequently, fluctuation of the tracking error signal occurs.
To solve this problem, the above-mentioned Patent Document 1 discloses a technique in which part of the main beam from the adjacent layer is diffracted by a hologram region provided in an optical member and the diffracted light does not enter the detector. The part of the main beam diffracted by the hologram region goes to the outside of a spot of the main beam in the detector. In consequence, a hole is formed in the spot of the main beam from the adjacent layer. The placement, shape, and so on of the hologram region are set such that the hole overlaps with the main detector and the sub-detectors. This prevents the main beam and sub-beams from the target layer from interfering with the main beam from the adjacent layer.
As another technique to solve the above-mentioned problem, there is disclosed a technique in which the area of a spot of the sub-beam in the recording layer of the optical disk is set larger than that of the spot of the main beam (see, for example, Japanese Patent Application Laid-open No. 2006-344380 (Paragraph [0036]) (hereinafter, referred to as Patent Document 2)). A grating pattern of a diffraction grating which generates three beams have predetermined regularity, whereby a spot of a sub-beam becomes larger. When the spot of the sub-beam becomes larger, the energy of the sub-beam is dispersed, and the interference between the sub-beam and the main beam from an adjacent layer is suppressed.